The invention relates generally to techniques for maintaining stability within an electrical circuit and, more particularly, to techniques for maintaining stability in a circuit under variable load conditions.
Some electrical circuitry is required to operate under a wide variety of different load conditions. For example, a voltage regulator circuit is typically required to maintain a regulated output voltage even when there is a widely varying load impedance coupled to the regulator. As can be appreciated, such circuitry must remain stable for all possible load conditions. The load impedance that is coupled to the circuitry can introduce a pole into the transfer function of the circuitry that is located within the active frequency range of the circuitry. In addition, the circuitry itself can also generate one or more poles that are within the active frequency range of the circuitry. For example, a frequency compensation capacitor within a circuit will often generate a dominant pole within the active frequency range of the circuit.
As is well known, the presence of two or more poles within the active frequency range of a circuit can lead to instability within the circuit unless additional measures are taken to prevent such instability. Typically, a zero is added to the transfer function of the circuitry to counteract the effect of having two poles within the active frequency range. The zero that is added to the transfer function will have a frequency that lies near that of one or both of the poles in a position that will prevent instability. In a circuit having a variable load impedance, however, the frequency of the load pole can vary greatly during normal circuit operation. Thus, while the fixed location of the zero may be optimal for some possible load conditions, it will generally be sub-optimal under other possible load conditions. Typically, the frequency of the zero will be chosen to provide optimal compensation for an intermediate load condition, thus compromising circuit stability under high and low load conditions.
Therefore, there is a need for a method and apparatus for maintaining the stability of a circuit under variable load conditions.